In previous studies with techniques developed in our laboratory we demonstrated that antibodies cross-reacting with mouse mammary tumor virus (MuMTV) are present in sera of breast cancer patients and that breast cyst fluids (BCF) have an IgA antibody with reactivity to MuMTV. The serum cross-reacting antibody appears to be specific for the deglycosylated membrane protein moiety p52 of MuMTV. The levels of antibody were highest in breast cancer sera of E. African patients (66%) and lowest in Chinese breast cancer sera (5%). Studies of families with high propensity of breast cancer showed that family members, including male members, have this antibody. We have now developed techniques to isolate gp52, to deglycosylate gp52 to p52, measure antigens cross-reactive with gp52 in breast cancer sera, and have demonstrated the presence of gp52 in breast cancer tissue. We have demonstrated that a human breast cancer cell line MCF7 which expresses cross-reacting gp52 of MuMTV on its surface can be lysed by human complement (C) and antibody to gp52 (determined by LDH release and trypan blue dye exclusion test). Breast cancer sera of 2 of 10 patients studied demonstrated the presence of the C-dependent cytotoxic antibody. We have prepared monoclonal antibodies to MuMTV, one of which (M.W. 53,000) gives a positive reaction with 15% of breast cancer tissue tested. In this proposal we plan to carry out population studies on the prevalence of "antigens," "antibodies" in human breast cancer sera. We will study the epidemiologic features of MuMTV-reactive components expression in breast cancer sera. We will determine the nature and specificity of the C-dependent cytotoxic antibody and characterize it. We will test the cytotoxic antibody using fresh breast tumor and develop more sensitive assays to detect it in blood. We will search for "antigens" to MuMTV, antibodies to MuMTV free in circulation and in circulating immune complexes (CICs) in sera of patients with breast cancer using monospecific antisera to MuMTV, gp52, p52, monoclonal antibodies. We will apply these reagents to study the relationship of the antibody in sera of breast cancer patients and presence of antigens recognized by antibodies to p52 in breast tissues from the same patients and to study particles which we have shown to be present in breast cyst fluid. We have expanded our geographical studies to Israel, Japan, Taiwan and Hawaii, and will continue our collaboration with China and Kenya. These studies will allow for a clearer definition of the relationship of MuMTV and human breast cancer.